Since car alarams that merely make noise are ignored by both thieves and passers-by, what we need is a new alarm system that will effectively prevent a car being stolen.

The idea is to have a car alarm that, upon being activated by suspicious activities, will do the following:
1. Say, in a high-quality
recorded voice that sounds plausibly authoritative, "Step away. Step away now!".
2. If the motion detector still senses the presence of the would-be thief, the alarm will restate the order, adding "or I will shoot you" in a voice that gets a little nervous towards the end.
3. With continued attention, the alarm will sound a single gunshot, as if a shot was fired in the air.
4. The alarm will sound three or four further "shots", this time with special effects squibs going off near where the thief is standing, that look as if the car was hit by bullets. The squib nearest the detected thief would also activate a red "blood" squib.

Obviously, this would require re-squibbing the car after every such attempt. It should still be cheaper in the long run, given the insurance savings.

Some riot-control vehicles in your more unrestive parts of
the world are said to have explosive 'subdual' charges built
into the exterior of the armored body. I see no reason why
this feature should not be adapted for the civilian market.

"STEP AWAY FROM THE VEHICLE OR YOU WILL BE SHOT
WITH A RUBBER BULLET... OKAY, YOU WERE WARNED...
SEE? HURTS, DOESN'T IT?"

Alterother, our local human-rights people rather insist on referring to them as "rubber-coated bullets". Something about people having a right not to be shot, no matter the particular ammunition. That's why I prefer faking it, less trouble with the lawyers.